As far as I can make out the chine of a barrel is the (max. Diameter) midpoint of the staves forming the barrel. If rolling the barrel, it would be rolled on it's chine.

The only definition I found does not really go much further:
humpenscrump "a musical instrument of rude construction." Alongside others like humstrum, celestinette and wind-broach, it was originally another name for the hurdy-gurdy.